The present invention relates to a hydraulic circuit for a hydraulic construction and working machine.
Conventionally, a construction and working machine is utilized not only for its primary purpose, but for multiple purposes owing to its good mobility and stability of a machine body.
Particularly in a hydraulic construction machine having a working device overhung from the machine body, the usage thereof tends to become diversified. FIG. 6 shows an example of a hydraulic shovel. The hydraulic shovel is primarily used for excavation of earth and sand at a level lower than a ground surface. A substance to be excavated contains hard and soft rock, earth and sand in a mixed condition, and the specific gravity of the substance is indefinite. Accordingly, an excavating resistance is fluctuated according to the substance, and an impact load is often applied to the machine. To prevent such a fluctuated load from adversely affecting the strengths of a machine body, boom 38, arm 39, bucket 40 and members related thereto, and also ensure a sufficient stability of the machine body during operation, the hydraulic shovel is provided with a limiting device for limiting an output and a holding power of hydraulic cylinders 14, 15 and 16 within a predetermined value.
In another kind of work such that the fluctuated load or the impact load is not applied in using the working device of the hydraulic shovel, e.g., in hoisting a heavy load on the ground or pulling a pile in the earth by rotating the boom 38 and the arm 39 to be operated by the hydraulic cylinders 14 and 16, respectively, there is no problem in the stability of the machine body and the strengths of the boom 38 and the arm 39 even if the operating forces of the hydraulic cylinders 14 and 16 are greater than those in the normal construction work. Such increased operating forces are rather advantageous for enlargement of a hoisting power or a pulling power, thus contributing to an improvement in capacity of the machine for this kind of work.
FIG. 3 is a diagram of an essential part of a hydraulic system in a general-purpose hydraulic prior art shovel of a construction and working machine. The operation of the hydraulic cylinder 14 for the boom 38 as shown in FIG. 6 will now be described. Referring to FIG. 3, a discharge pressure oil from a main pump 3 is allowed to flow into a hydraulic selector valve group A, and a main relief valve 23 is provided in a line leading from the main pump 3 to the hydraulic selector valve group A so as to limit the discharge oil pressure within a predetermined value, thereby protecting equipment in the hydraulic circuit and preventing excess outputs from various actuators. When a hydraulic selector valve 8 for the boom 38 is operated, the pressure oil admitted into the hydraulic selector valve group A serves to contract the hydraulic cylinder 14, wherein a contracting force of the hydraulic cylinder 14 is limited so as not to exceed a predetermined value in accordance with the set pressure of the main relief valve 23. Further, port relief valves 17 and 18 are provided in branch lines extending from lines leading from the hydraulic selector valve 8 to head-side and rod-side oil chambers of the hydraulic cylinder 14, so as to prevent breakage or deformation of boom 38 resulting from an abnormal stress generated by an external force greater than the above predetermined value. A set pressure of the port relief valves 17 and 18 is normally set to be slightly higher than that of the main relief valve 23, so that there may be no problem in a normal excavating operation by the boom 38, and only when an abnormal external force is applied to the boom 38, may the hydraulic cylinder 14 be freely expanded and contracted.
Although the above description is directed to the hydraulic cylinder 14 only with reference to FIG. 3, the same measures are provided for the other hydraulic cylinders 15 and 16.
However, in the hoisting or pulling work with the set pressure of each relief valve suitable for the excavating work, there occurs a problem that a working efficiency cannot be improved. To cope with this problem, the prior art has often taken the following measures. That is, as shown in FIG. 4, the main relief valve 23 is provided with a boosting pilot oil chamber 27 for boosting the set pressure of the main relief valve 23 by means of a pilot pressure. Furthermore, the set pressure of a port relief valve 17a leading to the load-side oil chamber of the hydraulic cylinder 14 is set to be higher than that in the normal excavating work in a range such that a static load stress generating in each part of the machine is permitted. In hoisting a heavy load or pulling a pile where no impact load is applied, a switch 30 located near a driving seat is operated as required to actuate hydraulic signal generating means 31 which in turn supplies a discharge pressure oil from a pilot pump 4 as a signal to the boosting pilot oil chamber 27.
Although the above description is directed to the hydraulic cylinder 14 for the boom 38, the same measures are taken for the other hydraulic cylinders 15 and 16 for the bucket 40 and the arm 39, respectively. According to the hydraulic circuit as shown in FIG. 4, the operating force of the hydraulic cylinders can be increased by an increased amount of the set pressure of the main relief valve 23 in the hoisting or pulling work generating no impact load, and a resisting force against an external force can of course be increased by an increased amount of the set pressure of the port relief valve 17a.
FIG. 5 is a diagram of an essential part of a hydraulic system as improved from the prior art shown in FIG. 4. Referring to FIG. 5, the port relief valve 17 leading to the load-side oil chamber of the hydraulic cylinder 14 is provided with a boosting pilot oil chamber 25 similar to the boosting pilot oil chamber 27 for the main relief valve 23. Both the boosting pilot oil chambers 25 and 27 are connected through pilot lines to the hydraulic signal generating means 31.
In such an improved hydraulic system, when the switch 30 is in an open state, the relief set pressures of the main relief valve 23 and the port relief valve 17 are maintained at the normal set pressures suitable for an excavating work of the hydraulic shovel, thus avoiding application of an excess load due to an external force. In the hoisting or pulling work generating no impact load as mentioned above with reference to FIG. 4, when the switch 30 is closed, the set pressures of the main relief valve 23 and the port relief valve 17 are increased to thereby increase the operating force of the hydraulic cylinder and the holding force in the oil chamber of the hydraulic cylinder 14 leading to the port relief valve 17.
In the construction machine employing the above-mentioned hydraulic circuit, there is a chance that an excavating force should be instantaneously increased during excavation as well as the hoisting or pulling work. In this case, the hydraulic signal generating means 31 may be operated by depressing an auto-return switch such as a push-button switch, wherein the operator must make sure of absolutely no application of an impact load; otherwise the strengths of the boom 38, the arm 39 and the bucket 40 must be increased so as to endure the increased excavating force, and the machine body must be so constructed as to endure a gravity of the strengthened working device.
As described above, the prior art hydraulic circuit for the construction and working machine is designed to increase the set pressure of the main relief valve by operating the switch or the push-button switch with the set pressure of the port relief valve previously boosted in hoisting a heavy load or pulling a pile or the like. However, when the engine is unintentionally stopped by any factors, or the operator erroneously opens the switch or releases his hand from the push-button switch during a hoisted condition of the heavy load, there is a danger that the heavy load will fall. Moreover, as the excavating work can be carried out with the set pressure of the main relief valve remaining increased, an excess force will be applied to the attachments at all times. Even if the set pressure of the main relief valve is not increased, the attachments inclusive of the boom, the arm and the bucket as well as the machine body will be adversely affected by an external force since the set pressure of the port relief valve is set to be higher than that in the normal excavating work.